CERN Courier

Fermilab's CDF experiment observes the Ωb–baryon

The CDF collaboration has announced the observation of a new particle, the Ωb– baryon, containing three quarks: two strange quarks and a bottom quark (ssb). The sighting of this "doubly strange" particle, predicted by the Standard Model, is significant because it strengthens physicists' confidence in their understanding of how quarks form matter. However, it conflicts with a result announced in 2008 by CDF's sister experiment, DØ.

The Ωb– is the latest entry in the "periodic table of baryons" illustrated in the figure. The Tevatron is unique in its ability to produce baryons containing the b quark, and the large data samples now available after many years of successful running have enabled experimenters to find and study these rare particles. The discovery of the Ωb– follows the first observations of two types of Σb baryons at the Tevatron in 2006 and the discovery there of the Ξb– baryon in 2007 (CERN Courier July/August 2007 p6).

Combing through almost 5 × 1011 proton–antiproton collisions produced by the Tevatron, the CDF collaboration isolated 16 examples in which the particles emerging from collisions reveal the distinctive signature of the Ωb–, which travels only a fraction of a millimetre before it decays into lighter particles. CDF has performed the first ever measurement of the Ωb–'s lifetime and obtained 1.13 + 0.53 – 0.40(stat.) ± 0.02(syst.) × 10–12 s.

In August 2008, the DØ experiment announced its own observation of the Ωb– based on a smaller sample of data from the Tevatron (CERN Courier November 2008 p7). Interestingly, the new observation from CDF conflicts with this earlier result. The CDF collaboration measures the mass of the Ωb– to be 6054.4 ± 6.8(stat.) ± 0.9(syst.) MeV/c2, compared with DØ's findings of 6165 ± 10(stat.) ± 13(syst.) MeV/c2. These two results are statistically inconsistent, leaving the teams from the two experiments wondering whether they are measuring the same particle. Furthermore, the experiments observed different rates of production for this particle. Perhaps most interesting is that neither experiment sees a hint of evidence for a particle at the mass value measured by the other.

Although the latest result announced by CDF agrees with theoretical expectations for the Ωb–, both in the measured production rate and in the mass value, further investigation is needed to solve the puzzle of these conflicting results.